Elastomeric yarns consist of single or multiple elastomeric fibers that are manufactured in fiber-spinning processes. By “elastomeric fiber” is meant a continuous filament which has a break elongation in excess of 100% independent of any crimp and which when stretched to twice its length, held for one minute, and then released, retracts to less than 1.5 times its original length within one minute of being released. Such fibers include, but are not limited to, rubbers, spandex or elastane, polyetheresters, and elastoesters. Elastomeric fibers are to be distinguished from “elastic fibers” or “stretch fibers” which have been treated in such a manner as to have the capacity to elongate and contract. Such fibers have modest power in contraction, and include, but are not necessarily limited to, fibers formed by false-twist texturing, crimping, etc.
For many years elastomeric fibers, such as spandex, have been covered with relatively inelastic fibers in order to facilitate acceptable processing for knitting or weaving, and to provide elastic composite yarns with acceptable characteristics for various end-use fabrics. The relatively inelastic fibers do not stretch and recover to the same extent as the elastomeric fibers. Examples of relatively inelastic yarns are synthetic polymers such as nylon or polyester. Within this specification, we will refer to the relatively inelastic fibers used for covering as “inelastic fibers” or “inelastic yarns”.
Several methods of covering elastomeric fibers with inelastic fibers are known and in use, including hollow-spindle covering, core spinning, air-jet entangling and modified false-twist texturing. Each method has its various advantages and disadvantages, and therefore is used selectively for various inelastic feed yarns, composite elastic yarns and end-use fabrics.
Air-jet entangling as a covering process for spandex elastomeric yarn is described in U.S. Pat. No. 3,940,917 (Strachan). A primary advantage of this process, when compared to the hollow-spindle covering process, for example, is the process speed at which the spandex can be covered with multifilament synthetic inelastic yarns. A typical process speed for hollow-spindle covering is up to 25 meters/minute, whereas a typical speed for air-jet entangling is 500 meters/minute or greater, or about 20 times or more as productive. Air-jet covered composite yarns have some deficiencies, however, as noted in Strachan; specifically, such composite yarns have loops extending from the covering component that partially obscure knitted stitch openings, resulting in a more opaque (versus transparent) look to knitted hosiery. Further, in knitted hosiery the extending loops increase the likelihood that difficulties will be encountered during the knitting operation and when the finished hosiery is in use. For example, the extending loops are more likely to be snagged or picked to cause a pulled strand when the hosiery is worn, resulting in a ruined garment. To attempt to address this problem, the Strachan patent teaches that using bicomponent yarns for the covering component can greatly improve knit stitch openness by activating the differential shrinkage and twisting of the bicomponent yarns during the hosiery dyeing and finishing processes. Using a bicomponent covering yarn, however, adds further expense, and the industry seeks a less expensive method to achieve improved knit stitch openness.
The elastic properties of composite elastic yarns made from prior art air-jet covering processes are determined primarily by the elastic properties and denier of the elastomeric feed yarn. Elastic properties are characterized by yarn mechanical stress-strain performance, and related characteristics such as elongation-to-break, tenacity-at-break, elastic modulus, and recovery force at various yarn elongation. These elastic properties in turn relate to fabric properties, such as physical dimensions, fabric stretch-unload power, and degree of compression or comfort in use.
The cost of an air-jet covered composite elastic yarn is determined primarily by the material cost of the elastomeric yarn included in the composite. The material cost of elastomeric yarn, in turn, is determined by the weight proportion of elastomeric yarn in the composite yarn, and by the cost per pound of the elastomeric yarn. Importantly, the cost per pound of elastomeric yarn depends upon the linear density, or denier, of the yarn; that is, fine denier or small diameter as-spun elastomeric yarn is typically much more costly on a per pound basis. For many stretch garment applications, a fine denier elastomeric yarn is used to form the composite yarn in order to achieve desired garment properties of stretch, recovery and comfort. During the covering process the elastomeric yarn is typically stretched, or drafted, to provide needed operating tension and to reduce its denier while it is being covered with the inelastic yarn. This is true not only for the air-jet process, but for all prior-art covering processes. Drafting the elastomeric yarn to a finer denier before forming the composite yarn reduces cost because the elastomeric feed yarn is of a higher-denier, lower-cost as-spun yarn. It follows that achieving ever-higher draft ratios in the covering process could lead to further cost reduction.
There have been limits, however, to the extent to which the elastomeric yarn can be drafted. For example, U.S. Pat. No. 3,387,448 (Lathem) shows that spandex may be drawn (stretched) to 500% (6×) of its original length and stabilized to a fine denier upon heat setting at oven temperatures between 180° F. to 700° F., and GB1,157,704 indicates that elastomer filaments may be drawn to 700% (8×) upon heating at oven temperatures up to 300° C., depending upon the heating oven type and residence time of the filament within the heater. See also, U.S. Pat. No. 6,301,760 (Beard). Hence, the industry continues to seek means for achieving higher draft ratios in elastomeric yarn covering processes.
Because of the variety of garments that are manufactured with elastic-covered yarns, and because of the different fabric stretch characteristics that are needed for various garment end uses, it would be very advantageous if an elastomeric yarn could be covered with an inelastic yarn at high speeds with an air-jet entangling process to form a composite yarn, while simultaneously modifying and tailoring the elastic properties of the resulting composite elastic yarn. In many cases for different garment applications, this ability could eliminate the need to change the denier and/or specification of the feed elastomeric yarn in the air-jet covering process, or to modify the composite-yarn elastic properties in a secondary process. Although it was known that the properties of elastomeric yarns can be altered by heat treatments, the art does not teach the means or the operating conditions needed to achieve desirable tailoring of composite yarn elastic properties, while simultaneously producing the composite yarn in an air-jet entangling process, with attention to reducing costs by using higher denier elastomeric yarns as the starting material and covering such elastomeric yarns with monocomponent inelastic yarns. The industry would benefit from a continuous, high-speed method to simultaneously produce an air-jet entangled, covered and heat-treated composite elastic yarn, wherein the method improved knit stitch openness using monocomponent inelastic covering yarns, and/or reduced the cost of said composite elastic yarns, as compared with prior air-jet covering methods, and/or desirably tailored the elastic properties of knit or woven fabrics from said composite yarns.